1. Technical Field
The present invention pertains to the direction of electromagnetic radiation towards a target and capture of electromagnetic radiation from a target; especially light.
2. Description of the Prior Art
Various telescope configurations can be used for communicating between terrestrially based terminals through a technique called free-space optical communications (FSOC). Generally, FSOC systems use a laser beam directed from a first opposing terminal and transmit that laser beam to a second opposing terminal. When the two laser beams lock on to each other, they provide two-way communication between the terminals. Of course, terminals need not be based terrestrially; one or both terminals involved in two-way communications can be space-borne. Also, communication can be only in one direction.
The selection of a telescope configuration influences almost all of the performance parameters in an FSOC system. For example, the size, weight, performance, and assembly complexity are all factors that contribute to system effectivity and deployment cost. The selection of the telescope type usually starts by trading off the performance offered by an unobscured telescope against the performance offered by an obscured system. Some of the earliest telescopes ever fabricated utilized refractive elements. These telescopes are very simplistic and can utilize commonly available optical components. Refractive telescope designs provide good overall optical performance and are easy to assemble. However, the materials used in refractive telescope technology are generally heavy, and can be more difficult to athermalize and correct for chromatic aberration. And most importantly, refractive telescopes are mechanically long in length, a trait incompatible with most portable applications. Infrared (IR) materials typically exhibit higher indices of refraction and can be made more lightweight. Use of such IR materials, though, requires additional care during alignment because visible alignment telescopes cannot be used. This can drive up the overall cost of system deployment.
Using reflective surfaces is another approach to forming a telescope. A popular telescope configuration comprises two reflective surfaces that fold an image path into a smaller and more compact package. There are two types of reflective telescope configurations known today; the on-axis configuration and the off-axis configuration. On-axis reflective designs can be easily manufactured and assembled and offer better control of aberrations compared to off-axis designs. There are many design forms that may be considered such as confocal paraboloids, Ritchey-Chrxc3xa9tien and classical Cassegrain types. Many manufacturing options are available such diamond turning, conventional polishing and replication. Substrate material options may also include glass types, metals and composite substrates. In the case of larger telescopes, a lightweighted approach can be employed to minimize the mass of the telescope assembly.
One significant difference between refractive and on-axis reflective configurations is that the reflective design has some of its incoming rays obscured. The effect of obscuring these rays becomes apparent when analyzing the optical throughput of an FSOC system. In these reflective configurations, the center-most rays entering the telescope are obscured from view. And because these, center-most rays have the greatest power density in a typical incoming Gaussian beam profile, much of the power transmitted by the sending side of an FSOC is lost. This makes obscured systems less desirable because of the reduced power efficiency vis-à-vis an unobscured telescope.
The throughput power efficiency of an unobscured telescope may allow the system to use a less powerful and less expensive laser. These types of cost benefits could easily be outweighed by the cost associated with manufacturing an unobscured refractive telescope assembly. Traditional reflective telescopes are cost-effective, but are less efficient optically. Herein lies the basic problem with the known art and the balancing of performance parameters that must be accomplished in any new FSOC system design.
There are reflective telescope designs that provide an unobscured aperture. These designs combine the compact nature of a reflective design with an unobscured aperture like a refractive design, but these are typically off-axis structures. Off-axis reflective designs have the advantage of offering an unobscured aperture. This enables maximum throughput of the Gaussian beam profile for the transmitted signal. Also, these designs have the potential to be athermalized and achromatized with the correct material selection. The off-axis approach suffers from increased field aberrations compared to on-axis systems, and can be more difficult to manufacture, assemble and align.
The structure of the telescope used in an FSOC is not the only system component that affects overall performance. A complete optical throughput budget for each FSOC generally considers several factors that contribute to optical loss. There are several sources of optical loss including material absorption, coating absorption and reflection, scattering, and free-space propagation loss. Polarization can also significantly affect the signal-to-noise ratio in an FSOC system if the design does not account for polarization. All the optical coatings in a system need to be cognizant of the polarization of the light that carries the data. Beam-splitting devices are especially sensitive to polarization effects.
Ideally, an FSOC system would benefit most if a telescopic device could be constructed that combined the unobstructed view offered by a refractive design together with the compact-size, reduced weight and low-cost of a reflective design. The present invention offers a unique combination of these features. Specifically, the present invention uses a reflective approach in order to collapse the overall size of the telescope. In contrast with traditional reflective designs, the present invention does not include a secondary reflector that can obscure the field-of-view. This one distinguishing feature, as embodied in the present invention, can be applied to FSOC systems or to other applications where electromagnetic radiation needs to be either directed to a remote target or collected from a remote source. It should be noted, that the method and apparatus of the present invention can be utilized throughout the entire electromagnetic spectrum, and that the scope of the present invention should not be limited to that portion of the electromagnetic spectrum comprising light.
Replacing the secondary reflector in the present invention is a specialized filter exhibiting special reflective properties. In a first example embodiment of the present invention, the filter is disposed in front of a primary reflector and allows electromagnetic radiation of a first polarization to pass through the filter. The filter reflects electromagnetic radiation of a second polarization. The effect of this selective reflection capability is to allow appropriately polarized radiation from a distant source to pass through the filter and strike the primary reflector. The primary reflector, having a focal length, reflects the polarized radiation toward the filter. This process of reflection reverses the polarization of the radiation. Because the radiation is now of a second polarization, it is reflected by the filter disposed in front of the primary reflector back toward the primary reflector. Generally, the filter is made sensitive to either right or left handed circularly polarized radiation. In most applications, electromagnetic radiation comprising light waves is utilized.
In a second example embodiment, the filter that is disposed in front of the primary reflector reflects light selectively according to the angle of incidence of the radiation striking the filter. Electromagnetic radiation incident on the filter substantially normal to the filter is allowed to pass through. Electromagnetic radiation that falls on the filter at an angle not normal to the filter is reflected.
In one example method for receiving electromagnetic radiation according to the present invention, electromagnetic radiation is allowed to pass through a filter toward a primary reflector. Once the electromagnetic radiation has propagated through the filter, it is then reflected by the primary reflector back toward the filter. The electromagnetic radiation is then reflected by the filter toward an aperture substantially in the center of the primary reflector.
In one alternative to this example method, reflection of the electromagnetic radiation by the filter is dependent on the angle at which the radiation strikes the filter. Generally, the filter is fabricated so as to provide high reflectivity when radiation strikes the filter at an angle greater than some cut-off angle. Radiation incident upon the filter at less than this cut-off angle is allowed to propagate through the filter.
In a second alternative to this example method, reflection of the electromagnetic radiation by the filter is dependent on the polarization of the electromagnetic radiation striking the filter. In this alternative method, the filter allows radiation incident upon the filter of a first polarization to propagate through the filter. Radiation of a second polarization is reflected by the filter.
The present invention further comprises a method for directing electromagnetic radiation toward some distant target. Radiation is injected through an aperture that is located substantially in the center of the primary reflector. This example method provides that the radiation be reflected by the filter toward the primary reflector. Once the radiation is reflected by the primary reflector, it is allowed to pass through the filter toward the target.
In one alternative embodiment of this method, the filter selectively reflects radiation based on the radiation""s angle of incidence. The filter is fabricated so as to provide high reflectivity when radiation strikes the filter at an angle greater than some cut-off angle. Radiation incident upon the filter at less than this cut-off angle is allowed to propagate through the filter.
In yet another alternative example method for directing electromagnetic radiation, the filter is sensitive to the polarization of the radiation. Electromagnetic radiation of a first polarization is again allowed to propagate through the filter. Electromagnetic radiation of a second polarization is reflected by the filter. In most embodiments of this method, the filter is sensitive to circularly polarized radiation.
The present invention further comprises an apparatus for directing radiation either to a distant target from a focal point or from a distant source toward some focal point. Embodying the method of the present invention, an example apparatus according to the present invention comprises a filter having an internal and external surface. The apparatus in this example embodiment further comprises a primary reflector that has a focal length and comprises an internal concave reflective surface and an aperture substantially in the center of the reflector. The reflector is disposed immediately behind the filter. In most embodiments, the filter is situated at a distance from the primary reflector that is one-half the focal length of the primary reflector. In some embodiments, a collimating lens is disposed behind the aperture in the primary reflector for the purpose of re-collimating light gathered by the telescope.
In one alternative example embodiment of the present invention, the filter comprises a cholesteric liquid crystal manufactured so as to be reflectively sensitive to the polarization of radiation incident thereon. The present invention provides for two different physical structures to support the cholesteric liquid crystal. In the first of these, the cholesteric liquid crystal is sandwiched between two flat pieces of glass. In the second, and preferred support structure, the cholesteric liquid crystal is deposited onto a substrate. It should be noted that the scope of the present invention should not be limited to the manner in which the cholesteric liquid crystal is supported. In some embodiments that comprises the cholesteric liquid crystal, an internally-aligned quarter-wave retarder is disposed immediately behind the primary reflector concentric with the aperture therein. The purpose of the quarter-wave retarder is to accept linearly polarized radiation from a source and transform it into circularly polarized radiation. The circularly polarized radiation can then be reflected by the filter back toward the primary reflector. As the circularly polarized radiation is reflected by the primary reflector, it is converted from one direction of circular polarization to the other allowing it to pass through the filter instead of being reflected.
In another alternative example, either a bandpass filter or a short pass filter comprises the present invention. Either the bandpass or short pass filters are constructed so as to make the filter reflectively sensitive to the angle of incidence at which radiation strikes the filter. The support mechanisms for the bandpass or short pass filter is analogous to that used for the cholesteric liquid crystal. The bandpass or short-pass filter can be sandwiched between two flat pieces of glass or deposited onto a substrate. Again, the support mechanisms used to hold the bandpass or short-pass filter in place relative to the primary reflector should not be considered as limiting the scope of the present invention.
In some embodiments of a radiation directing device according to the present invention comprising either the cholesteric liquid crystal filter, the bandpass filter, or the short pass filter the directing device itself may further comprise either an emitter or a detector disposed at the aperture comprising the primary reflector. In some alternative embodiments, the emitter or the detector may be disposed remotely from the primary reflector and coupled to the aperture using a radiation channel. Depending on the wavelength of electromagnetic radiation that a particular radiation directing device has been designed to operate with, the radiation channel may be an optical fiber.
In another example embodiment of the present invention, a beam-splitter may further comprise the present invention. In this case, the beam-splitter is disposed immediately behind the primary reflector. An electromagnetic radiation emitter and an electromagnetic radiation detector are coupled to the beam-splitter. This provides for the capability of simultaneously receiving or transmitting electromagnetic radiation using the device of the present invention.